Chronic hypoxemia is a leading cause of fetal morbidity and mortality. The fetus redistributes its cardiac output during hypoxemia to the heart by adaptive mechanisms that are poorly understood. We hypothesize that fetal hypoxemia alters the expression of oxygen-sensitive genes such as nitric oxide synthase (NOS) in the vascular endothelium and alters the regulation of coronary blood flow. We propose that chronic hypoxemia is a stimulus for initiating programming of oxygen-sensitive genes that alters coronary reactivity and flow regulation in the fetus. Permanent changes in the expression of oxygen-sensitive genes during fetal hypoxemia may be a mechanism by which fetal programming causes sustained changes in cardiovascular function after birth. Fetal hypoxemia is induced by placing pregnant guinea pigs in a hypoxic chamber (12 percentO2) for 14 days. Coronary artery reactivity to agonist stimulation is measured in isolated perfused fetal hearts from normoxic and hypoxemic animals. Gene expression and distribution of eNOS/iNOS in endothelial, vascular smooth muscle, and cardiac cells will be determined by immunocytochemistry and nonradioactive in situ hybridization. Laser Capture Microdissection of individual cells will be applied to heart sections for measure of cell specific NOS mRNA. To test whether altered fetal responses are sustained postnatally, coronary artery responses and NOS gene expression will be studied in hearts from adolescent animals previously exposed to hypoxemia in utero. Aim 1: To test the hypothesis that chronic hypoxemia alters NO-dependent mechanisms of the fetal coronary microcirculation. Aim 2: To test whether chronic hypoxemia increases eNOS and iNOS gene expression in endothelial, vascular and cardiac cells of the fetal heart. Aim 3: To test whether chronic hypoxemia permanently alters coronary artery flow responses after birth as a mechanism of fetal programming. This proposal will provide novel information on how chronic hypoxemia affects endothelial-dependent mechanisms in the regulation of fetal coronary flow and aid in our understanding of how the coronary circulation adapts to chronic hypoxemia in utero and postnatally.